Electronic gate enhancement of Schottky junction solar cells

ABSTRACT

Various systems and methods are provided for Schottky junction solar cells. In one embodiment, a solar cell includes a mesh layer formed on a semiconductor layer and an ionic layer formed on the mesh layer. The ionic layer seeps through the mesh layer and directly contacts the semiconductor layer. In another embodiment, a solar cell includes a first mesh layer formed on a semiconductor layer, a first metallization layer coupled to the first mesh layer, a second high surface area electrically conducting electrode coupled to the first metallization layer by a gate voltage, and an ionic layer in electrical communication with the first mesh layer and the second high surface area electrically conducting electrode. In another embodiment, a solar cell includes a grid layer formed on a semiconductor layer and an ionic layer in electrical communication with the grid layer and the semiconductor layer.

STATEMENT AS TO FEDERALLY SPONSORED RESEARCH

The invention was made with government support under agreementECCS-0824157 awarded by the National Science Foundation. The U.S.government has certain rights in the invention.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the 35 U.S.C. §371 national stage of PCT applicationPCT/US2011/034107, filed Apr. 27, 2011, which claims priority to and thebenefit of U.S. provisional application entitled “ELECTRONIC GATEENHANCEMENT OF SCHOTTKY JUNCTION SOLAR CELLS” having Ser. No.61/328,417, filed Apr. 27, 2010, both of which are hereby incorporatedby reference in their entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present disclosure can be better understood withreference to the following drawings. The components in the drawings arenot necessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present disclosure. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a graphical representation of an example of a solar cell inaccordance with various embodiments of the present disclosure.

FIGS. 2A and 2B are examples of energy band diagrams illustrating theenergy levels of the metal and the semiconductor included in the solarcell of FIG. 1 in accordance with various embodiments of the presentdisclosure.

FIG. 3 is a graph of an example of simulated current density versusvoltage (J-V) characteristics for an embodiment of the solar cell ofFIG. 1 in accordance with various embodiments of the present disclosure.

FIGS. 4 and 5 are graphs of an example of J-V characteristics measuredon an embodiment of the solar cell of FIG. 1 while illuminated inaccordance with various embodiments of the present disclosure.

FIG. 6 is a graphical representation of another example of a solar cellin accordance with various embodiments of the present disclosure.

FIG. 7A is a graphical representation of another example of a solar cellin accordance with various embodiments of the present disclosure.

FIG. 7B is a picture of a grid layer of the solar cell of FIG. 7A inaccordance with various embodiments of the present disclosure.

FIG. 8 is a graph of an example of J-V characteristics while dark andilluminated for an embodiment of the solar cell of FIG. 1 without anionic conductor layer and an embodiment of the solar cell of FIG. 7Awithout an ionic conductor layer in accordance with various embodimentsof the present disclosure.

FIG. 9 is a graph of an example of J-V characteristics an embodiment ofthe solar cell of FIG. 7A without and with an ionic conductor layer inaccordance with various embodiments of the present disclosure.

FIG. 10A is a graph of an example of experimental J-V characteristics anembodiment of the solar cell of FIG. 7A in accordance with variousembodiments of the present disclosure.

FIG. 10B is a graphical representation of the simulation parameters andgeometry of the solar cell of FIG. 7A in accordance with variousembodiments of the present disclosure.

FIGS. 10C-10H are graphical representations illustrating examples of theelectric field developed in the depletion layer of the solar cell ofFIG. 7A in accordance with various embodiments of the presentdisclosure.

DETAILED DESCRIPTION

Solar cells are useful for converting sunlight into energy. The presentdisclosure describes solar cells including a nanotube film-semiconductorjunction, which has a built-in potential that is not merely a functionof material properties but also can be modified by electronic gating.Additionally, electronic gating can modify the interface dipole at thejunction between the nanotube film and the semiconductor, reducingbarriers to charge transport across the junction. Furthermore, theelectronic gating can contribute to the electric field across thedepletion layer enhancing the efficiency with which charges are sweptout of the region further boosting the power generation capabilities.

FIG. 1 is a perspective view of a nonlimiting embodiment, among others,of a solar cell 100, denoted as solar cell 100 a. Solar cell 100 a ishelpful for demonstrating the functionality of the various embodimentsdescribed herein. Embodiments of solar cells 100 more useful incommercial applications are described in detail below. The solar cell100 a includes a back contact layer 101 and a semiconductor layer 102.The solar cell 100 a further includes an insulating layer 103 on thesemiconductor layer 102. A portion of the insulating layer 103 is etchedto expose the semiconductor layer 102, which is surrounded by a firstmetallization layer 104 a. A first mesh layer 106 a is placed on theetched portion such that the first mesh layer 106 a contacts thesemiconductor layer 102 and the first metallization layer 104 a. Thefirst mesh layer 106 a is an electrically conducting, porous mesh layertransparent to an appreciable fraction of the solar radiance spectrum.

The solar cell 100 a further includes a second metallization layer 104 bon the insulating layer 103. A second mesh layer 106 b contacts thesecond metallization layer 104 b. However, in other embodiments of asolar cell 100, no insulating layer 103, no second metallization layer104 b and no second mesh layer 106 b is included.

The mesh layers 106 a and 106 b are electrically isolated from eachother. The first metallization layer 104 a forms a contact for the firstmesh layer 106 a, and the second metallization layer 104 b forms acontact for the second mesh layer 106 b. Also, the first mesh layer 106a contacts the semiconductor layer 102 whereas the second mesh layer 106b is isolated from the semiconductor layer 102 by the insulating layer103. The first mesh layer 106 a and first metallization layer 104 acollectively form a junction electrode. Similarly, the second mesh layer106 b and the second metallization layer 104 b collectively form a gateelectrode.

In the solar cell 100 a depicted in FIG. 1, the first mesh layer 106 aand the second mesh layer 106 b are approximately the same size and,thus, have comparable surface areas. However, in other embodiments, thefirst mesh layer 106 a and the second mesh layer 106 b may be differentshapes but have similar surface areas. Also, in embodiments of a solarcell 100 where no second mesh layer 106 b is included, the junctionelectrode and the remote gate electrode have approximately the samesurface areas.

An ionic conductor layer 108 covers at least a portion of each of thefollowing layers: the first mesh layer 106 a, the second mesh layer 106b, the first metallization layer 104 a, the second metallization layer104 b, and the insulating layer 103. The ionic conductor layer 108 alsoseeps down through the first mesh layer 106 a and also directly contactsthe semiconductor layer 102. The first metallization layer 104 a and thesecond metallization layer 104 b are biased by a voltage source 120 at agate voltage V_(G).

In operation, a solar cell 100 is a source of power. The solar cell 100a illustrated in FIG. 1 generates power when the surface of thesemiconductor layer 102 is illuminated by solar radiation. Specifically,solar radiation (hv) passing through the ionic conductor layer 108 andthrough the first mesh layer 104 a causes an accumulation of charge onthe first metallization layer 104 a and the opposite charge on the backcontact layer 101. Under illumination, the potential difference betweenthe first metallization layer 104 a and the back contact layer 101 whenno resistive load is electrically connected to the first metallizationlayer 104 a and the back contact layer 101 is called the open circuitvoltage V_(OC).

When a resistive load that is capable of consuming power is electricallyconnected to the first metallization layer 104 a and the back contactlayer 101, a current will flow through the load. Once current flowsthrough the load, the voltage between the first metallization layer 104a and the back contact layer 101 will decrease. The product of thevoltage across the load and the current flowing through the load is thepower dissipated by the load and is also the power being generated bythe solar cell 100 a. For fixed illumination of the solar cell 100 a, aplot of the current through the load versus the voltage across the load,as the load resistance changes permits extraction of the figures ofmerit that characterize the performance of the solar cell 100 a.Alternatively, a sourcemeter can apply bias voltages between the firstmetallization layer 104 a and the back contact layer 101, whilemeasuring the current. A plot of this current versus the applied biasvoltage also permits extraction of the figures of merit thatcharacterize the performance of the solar cell 100 a.

Having described the structure and power generaing capabilities of thesolar cell 100 a in the embodiment in FIG. 1, various materials that maybe included in the layers of the solar cell 100 a will be described. Thesemiconductor layer 102 includes one or more of the followingsemiconductor materials: Si, Ge, and/or GaAs, CdS, ZnO, CdSe, TiO₂. Thesemiconductor material is doped, and in some embodiments thesemiconductor material is n-type Si.

The back contact layer 101 includes one or more metals and/or an alloythereof that forms a low resistance Ohmic contact with the semiconductorlayer 102. When the semiconductor layer 102 includes lightly doped n-Si,the back contact layer 101 may include, for example, titanium, aluminum,and/or indium gallium eutectic. Alternatively, the back side of thesemiconductor layer 102 (i.e., the side of the semiconductor layer 102that contacts the back contact layer 101) may be heavily doped, in whichcase most metals will form suitable low resistance contacts.

In embodiments that include an insulating layer 103, such as the solarcell 100 a depicted in FIG. 1, the insulating layer 103 may include aninsulating material such as silicon dioxide (SiQ). As will be discussedin further detail, some embodiments do not include an insulating layer103.

Similarly, the first metallization layer 104 a and the secondmetallization layer 104 b of the solar cell 100 a illustrated in FIG. 1may include one or more of the following conductive materials: chromium(Cr), gold (Au), palladium (Pd), and indium tin oxide (ITO). However, aswill be discussed in further detail below, some embodiments of a solarcell 100 do not include metallization layers 104 a, 104 b.

The first mesh layer 106 a (and the second mesh layer 106 b inembodiments that include a second mesh layer 106 b) includes one or moreof the following materials: single wall carbon nanotubes, double wallcarbon nanotubes, multi-wall carbon nanotubes, graphene, semiconductingnanowires, metallic nanowires, a metallic grid or a semiconducting grid.Regardless of the material, the first mesh layer 106 a, which is incontact with the semiconductor layer 102, is sufficiently transparent tothe solar radiation that the major fraction of the radiation reaches thesurface of the semiconductor layer 102. Furthermore, the first meshlayer 106 a is electrically conducting with a sheet resistance of about300 Ohms/sq or less. Additionally, the first mesh layer 106 a issufficiently porous to permit access of the ionic conductor layer 108 toboth the surface of the semiconductor layer 102 and the portions of thefirst mesh layer 106 a that contact the surface of the semiconductorlayer 102.

For example, in the embodiment of a solar cell 100 a illustrated in FIG.1, the first mesh layer 106 a and the second mesh layer 106 b eachinclude a network of nanotubes. Because the first mesh layer 106 a andthe second mesh layer 106 b include a network of nanotubes, the firstmesh layer 106 a and the second mesh layer 106 b are porous andtransparent. Nanotubes have a low density of states but a highconductivity. The Fermi level of nanotubes can also be adjusted bydoping using an electron acceptor or an electron donor. It can beenergetically favorable for carbon to donate an electron or accept anelectron to form an ionic bond. Accordingly, in some embodiments, thefirst mesh layer 106 a and/or the second mesh layer 106 b is p-dopedusing electron acceptors such bromine, thionyl chloride, or nitric acid.Further, the first mesh layer 106 a and/or the second mesh layer 106 bmay be roughly a third metallic and the remaining two thirdssemiconducting, but with doping of semiconducting nanotubes, they becomeconductive enough to be considered as effectively all metallic.

The ionic conductor layer 108 includes an ionically conducting material.This ionically conducting material possesses a high ionic conductivityand is transparent to an appreciable fraction of the solar irradiancespectrum. For example, the ionically conducting material may be an ionicliquid, an electrolyte solution, or a solid state electrolyte. In someembodiments, the ionic liquid is 1-Ethyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide (EMI-BTI). The voltage applied by thegate voltage 120 remains below the reduction-oxidation (redox) potentialof the ionically conducting material to avoid electro-chemicalalteration of the properties of the ionically conducting material.

Referring to the solar cell 100 a illustrated in FIG. 1, at steadystate, the voltage applied by the gate voltage 120 to the metallizationlayers 104 a, 104 b will inject charge of opposite sign onto the meshlayers 106 a and 106 b. For example, if 0.5 V is applied between thefirst mesh layer 106 a and the second mesh layer 106 b while the secondmesh layer 106 b is made negative, then electrons will be with drawnfrom the nanotubes in the first mesh layer 106 a (leaving the first meshlayer 106 a positively charged) while the second mesh layer 106 b willhave electrons injected, making it negatively charged. In the absence ofthe ionic conductor layer 108, the capacitance of this arrangement israther small so that not much charge can be removed from the first meshlayer 106 a and put onto the second mesh layer 106 b.

The Coulombic attraction of the positive charge induced on the firstmesh layer 106 a for the remaining electrons on the first mesh layer 106a is too strong for more electrons to be removed by the power supply atthis voltage. When the ionic conductor layer 108 is present, however,the capacitance becomes much larger so that much more charge can beremoved from the first mesh layer 106 a, to be put onto the second meshlayer 106 b at this same voltage. This occurs because in response to theinitial positive charge left on the first mesh layer 106 a, negativeions from the ionic conductor layer 108 migrate to the surfaces of thenanotubes. These negative ions compensate the Coulombic attraction ofthe positive charge on the nanotubes for its remaining electronsallowing the power supply to withdraw more electrons from the first meshlayer 106 a at this same voltage. Positive ions attracted to thenegative charge on the second mesh layer 106 b similarly allows manymore electrons to be injected onto the nanotubes in that layer. Thepresence of the ionically conducting material thus permits appreciablechanges in the electronic population of the nanotubes in the mesh layers106 a, 106 b. Because a charge transfer dopant similarly changes theelectronic population of a nanotube this can be considered an electronicdoping of the nanotubes. The change in their electronic populationscauses the Fermi level of the mesh layers 106 a, 106 b to change. Thecharges on the nanotubes do not come from the ionic conductor layer 108,but rather from the power supply 120, however the ionic conductor layer108 allows the charges to accumulate on the mesh layers 106 a, 106 b.Therefore, the embodiment of a solar cell 100 a illustrated in FIG. 1 isa device that enables changing the Fermi level of a nanotube layerelectronically.

As mentioned above, the voltage applied by the gate voltage 120 remainsbelow the reduction-oxidation (redox) potential of the ionicallyconducting material of the ionic conductor layer 108, and therefore, nocurrent flows at steady state (once charge reorganization has settled).This means that under steady operation, the applied gate voltage neitherconsumes nor supplies power. In other words, since power is equal to theproduct of current and voltage, there is no power drawn because there isno current flowing. The embodiment of a solar cell 100 a illustrated inFIG. 1 has a power conversion efficiency of about 11%.

Various embodiments of the solar cell 100, including solar cell 100 a,include a metal to semiconductor junction. For example, the first meshlayer 106 a and the semiconductor layer 102 form a metal tosemiconductor junction. FIGS. 2A and 2B are energy band diagrams showingthe energy levels of the metal and the semiconductor before (FIG. 2A)and after (FIG. 2B) the metal and semiconductor are placed into contact.FIGS. 2A and 2B are simplified pictures that ignore (for the moment)surface states existing or induced at the surface of the semiconductor.The Fermi levels in the metal and semiconductor characterize theiraverage highest filled electronic energy states in isolation.

A workfunction φ corresponds to the energy required to take an electronfrom the Fermi level to vacuum far from the material. The case shown isthe one where the workfunction of the metal possesses a workfunction,φ_(M), relative to the workfunction of the semiconductor for which|φ_(M)|>|φ_(S)|. This is relevant to the junction between the first meshlayer 106 a and the semiconductor layer 102 of the solar cell 100 aillustrated in FIG. 1 with the first mesh layer 106 a taking the role ofthe metal and where the semiconductor layer 102 includes n-Si.

When the nanotubes of the first mesh layer 106 a and the n-Si are placedinto contact, thermodynamic equilibration causes their Fermi-levels toalign. This is accomplished by a transfer of electrons from the n-Si tothe first mesh layer 106 a. The result of this equilibration is shown inFIG. 2B. The electrons transferred from the n-Si conduction band resultin a region of effectively intrinsic Si at the junction, which possessesno free carriers (i.e., no free electrons in the conduction band orholes (absence of electrons) in the valence band). This effectivelyintrinsic region is called the depletion region having a characteristiclength called the depletion width. The width of the depletion regiondepends in part on the doping density of the semiconductor material inthe semiconductor layer 102.

The additional potential energy of an electron in the region of thisdepletion layer is reflected in the band bending in the n-Si near theinterface. Energetically, the free electrons in the bulk of the n-Sioccupy a narrow strip just above the conduction band minimum. After thecontact of the two materials is made the bent conduction band in then-Si reflects an energy barrier that electrons in the conduction band ofthe n-Si (gray region to the right of the bent bands) must overcome toget from the n-Si to the first mesh layer 106 a. The approximate heightof this barrier is called the built-in-potential V_(bi) and themagnitude of the built in potential (in the absence of surface states)is given by V_(bi)=φ_(S)−φ_(M). The barrier for electrons going from thenanotubes to the n-Si is called the Schottky barrier and isapproximately the difference between the original conduction bandminimum in the n-Si and the Fermi level of the nanotubes. The junctionof the two materials is called a Schottky junction.

As stated above, in the embodiment illustrated in FIG. 1, the first meshlayer 106 a contacts the semiconductor layer 102. The junction of thefirst mesh layer 106 a with the semiconductor layer 102 forms a Schottkyjunction 200 (i.e., a metal-semiconductor junction). FIGS. 2A and 2B arean energy band diagram illustrating energy levels of the Schottkyjunction 200 in the absence of a voltage applied by the gate voltage120.

When a photon is incident onto a semiconductor, if the photon energyexceeds the bandgap energy of the semiconductor, the photon can beabsorbed, with the energy going into the promotion of an electron fromthe valence band of the semiconductor to its conduction band (leavingbehind a hole in the valence band). In the absence of other phenomenathe electron will ultimately decay back to the valence band, theelectron recombining with a hole, and the energy dissipated as radiationor lattice vibrations (heat). If, however the photon is absorbed withinthe depletion region of a nanotube/n-Si Schottky junction 200, or if theelectron-hole pair generated by the photon outside the depletion regioncan diffuse into this region, then the band bending provides anelectromotive force that sweeps electrons away from the Schottkyjunction 200, toward the bulk n-Si, while holes in the valence band areswept towards the nanotube side of the junction. In this manner thebuilt-in potential resulting from the band bending powers the device tocreate a solar cell 100. The resulting device is a nanotube/n-SiSchottky junction 200 solar cell 100. The reported power conversionefficiency from such solar cells 100 is about 7%.

When the solar cell 100 a illustrated in FIG. 1 is exposed to light (hv)as shown, the photons are transmitted through the first mesh layer 106 aand absorbed within the underlying depletion region in the semiconductorlayer 102. This absorption generates electron-hole pairs that are drivenin opposite directions by the built-in potential V_(bi). The holes areextracted in the first mesh layer 106 a, and the electrons are extractedin the semiconductor layer 102.

In the absence of illumination (i.e., in the dark), when a voltage isapplied to the Schottky junction 200, the Schottky junction 200 behavesas a diode. For example, when the Schottky junction 200 isforward-biased (i.e., a positive charge is applied to the first meshlayer 106 a and a negative charge is applied to the semiconductor layer102), the negative charge applied to the semiconductor layer 102 fromthe power supply raises and unbends the initially bent bands. As thebands approach the flat band condition, the barrier to transport acrossthe Schottky junction 200 decreases and forward current can begin toflow, growing exponentially with further increases in the forward biasvoltage. When reverse bias voltage is applied to the Schottky junction200 (i.e., a negative charge is applied to the first mesh layer 106 aand a positive charge is applied to the semiconductor layer 102) thefurther electrons withdrawn from the depletion region serves to increasethe depletion region width, increases the amount of band bending andincreases the barrier to transport so that only very small currents canflow in the reverse direction.

The figures of merit for a solar cell 100 can be extracted from thecurrent density—voltage plots of a solar cell 100 exposed to the solarspectrum at a standard illumination intensity. FIG. 3 is a graph 300 ofa simulated current density versus voltage (J-V) plot for an embodimentof a solar cell 100 including a Schottky junction 200. The voltagerepresented on the x-axis is the bias voltage Vg applied to theilluminated solar cell 100 while the current density is the resultingcurrent divided by the area of the illuminated junction of solar cell100. The Prince model of the current density-voltage relation for asolar cell 100 includes series and shunt resistances. For typicallyattained values, the shunt resistance has a negligible effect on the J-Vcharacteristic whereas the series resistance (Rs) has profound effects.FIG. 3 illustrates the J-V characteristics of three curves havingdifferent series resistances, R_(S)=0 ohms-cm², R_(S)=20 ohms-cm², andR_(S)=40 ohms-cm². R_(S)=0 ohms-cm² may be thought of as an ideal case,yielding maximum power, where power (i.e., P=IV) is indicated by thearea of the boxes in the fourth quadrant (IV) of FIG. 3. The relativeareas of the boxes associated with the different curves illustrate thedeleterious effect of series resistance on the performance of a solarcell 100.

For a solar cell 100 in the dark, the forward-electron current becomesappreciable when the applied forward bias voltage counteracts thebuilt-in potential V_(bi). Referring to FIG. 2, under these conditions,the energy bands in the semiconductor layer 102 are raised and flattenedsufficiently to permit forward tunneling and thermionic currents,referred to as a forward-electron current. When the solar cell 100 isilluminated, the J-V curve additionally includes a counter-propagatingphoto-electron current. Referring to FIG. 3, the voltage at which thenet current is zero corresponds to the applied bias voltage at which theforward diode current equals the photocurrent flowing in the oppositedirection. Since the forward current requires a flattening of the bands,while the electromotive force for photo-carrier separation is providedby the bent bands, the voltage at which these currents are equal, i.e.,the open circuit voltage, V_(OC), (approaching the flat band condition)provides a sensitive measure of the built-in potential.

FIG. 4 is a graph 400 of current density versus voltage (J-V) measuredon the embodiment of solar cell 100 illustrated in FIG. 1 at variousbiases for the gate voltage V_(G) while the solar cell 100 isilluminated. The various biases for the gate voltage V_(G) are indicatedby the table inset 410 in FIG. 4. The voltage represented on the x-axisis the solar cell 100 bias voltage V_(B). As can be seen in FIG. 4, whenthe voltage applied by the gate voltage V_(G) is positive, theperformance of the solar cell 100 is degraded and an increase inprominence of a “kink” (energy gap feature) 430 occurs when the solarcell bias voltage V_(G) is close to or about 0 V (i.e., the open circuitvoltage, V_(OC)). In contrast, negative voltages applied by the gatevoltage V_(G) enhance the performance of the solar cell 100 and reducethe prominence of the “kink” 430. FIG. 5 is a graph of the currentdensity versus voltage (J-V) presented in FIG. 4, except over a narrowerrange of solar cell bias voltages V_(B). Accordingly, FIG. 5 isessentially a magnified view of the open circuit voltages V_(OC)illustrated in FIG. 4.

Table 1 below describes various solar cell 100 a characteristicsextracted from the J-V curves illustrated in the graphs 400 of FIGS. 4and 5 at the different gate voltages. Since the formulas for calculatingthese various figures of merit are known to persons skilled in the art,an explanation of those formulas is omitted here for purposes ofbrevity.

TABLE 1 Solar cell characteristics from the gated J-V curves of FIGS. 4and 5. Gate Bias (V) −0.75 −0.45 −0.15 0.0 +0.15 +0.45 +0.75 V_(OC)(V)0.55 0.53 0.51 0.49 0.47 0.41 0.33 J_(SC)(mA/cm²) 25.0 25.3 25.2 25.025.0 24.9 24.8 FF 0.79 0.77 0.71 0.68 0.62 0.54 0.44 PCE(%) 10.9 10.59.2 8.4 7.4 5.5 3.6

The changes in open circuit voltage V_(OC) illustrated in FIGS. 4 and 5are consistent with a change in built-in potential V_(bi) indicated inthe band diagram inset 420 in FIG. 4. As illustrated in the band diagraminset 420, negative gate voltages withdraw electrons from the first meshlayer 106 a, causing the Fermi level of the first mesh layer 106 a toshift further from the vacuum level relative to the semiconductor layer102. The Fermi level equilibration results in a greater built-in voltageV_(bi), reflected in a correspondingly greater open circuit voltageV_(OC).

In addition to the changes in the built-in voltage V_(bi) as the gatevoltage V_(G) changes, the series resistance R_(S) changes as well.Specifically, as depicted in FIG. 4, the series resistance R_(S)increases as the gate voltage V_(G) increases, which is indicated by themore shallow slopes of the various curves at higher currents. Thischange in series resistance occurs due to electrolyte-induced changes inthe first mesh layer 106 a caused by the ionic conductor layer 108. Theembodiment of the solar cell 100 a measured includes a first mesh layer106 a that is one third metallic nanotubes and the remaining two thirdssemiconducting nanotubes. Accordingly, the changes in series resistanceoccur as the Fermi levels of the nanotubes in the first mesh layer 106 aare pushed by the gate voltage V_(G) into (or out of) the band gap ofthe nanotubes, effectively switching off (or on) the conductance in thesemiconducting nanotubes, which changes the resistivity of the firstmesh layer 106 a thus explaining the modified slopes in the “linear”regions of the various curves with the changing gate voltage V_(G).

Neither a shift in built-in voltage V_(bi) or a modified seriesresistance Rs can explain the low current “kink” 430 in FIG. 4 nearV_(OC) and its increasing prominence with increasing gate voltage V_(G).However, this “kink” 430 can be explained in terms of the behavior ofthe first mesh layer 106 a and the semiconductor layer 102 at theirrespective surfaces where each layer contacts the other. Schottkybarrier models of a metal-semiconductor junction, which will bediscussed in the following paragraphs, are useful for providing afoundational understanding of this “kink” 430.

According to the Schottky-Mott model of a Schottky junction 200(discussed above), the built-in potential V_(bi) depends only on thedifference in the pre-contact Fermi-levels of the metal and thesemiconductor according to the expression: V_(bi)=φ_(S)−φ_(M). Thismodel is only applicable to cases in which surface states can beignored. According to the Bardeen model of a Schottky junction 200, atmetal-semiconductor junctions, the termination of a bulk semiconductormaterial at its surface leads to surface states. Treated as a continuum,these states have their own energy dependent density of states, and theenergy distribution of that density depends on the particular crystalface involved, surface atomic reconstruction, defects, and impurities.Because the bulk of a semiconductor material must be in thermodynamicequilibrium with its own surface, the spatial distribution of chargebetween the surface and the bulk can itself lead to an intrinsic bandbending and associated depletion layer even before a metal is broughtinto contact with the semiconductor surface.

If the surface states have a band of high density around the highestoccupied surface state, then thermodynamic equilibration with the Fermilevel of the contacting metal in the metal-semiconductor junction occursvia electrons exchanged with this high density band of surface states.Accordingly, there is relatively little change in the band bending uponmaking the metal-semiconductor contact, and the Schottky barrier isindependent of the work function of the metal in the metal-semiconductorjunction, effectively “pinning” the Schottky barrier height and thebuilt-in potential V_(bi), independent of the metal contacted to thesemiconductor.

The Schottky-Mott and Bardeen models comprise the opposite extremes ofwhat occurs at Schottky junctions 200. To allow for a degree ofdependence on the metal workfunction, modern Schottky barrier modelsincorporate the idea of surface states at the semiconductor surface andan additional interface dipole that occurs between the metal and chargetransferred into these surface states upon contacting the semiconductor.The interface dipole may be associated with bond polarization across thechemical bonds between the metal and the semiconductor and/or the chargetransferred by energy equilibration between surface states in thesemiconductor and the metal. This interface dipole is assumed to bethin, and the dipole functions as a tunneling barrierwhose effect isfolded into the Schottky barrier height. The modern view ofmetal-semiconductor junctions thus allows for modulation of the built-inpotential mediated by charge exchange with surface states, combined withan interface dipole that contributes to the Schottky barrier height.

Referring again to FIGS. 4 and 5, the “kink” 430 results from agate-modulated enhancement and suppression of the interface dipole atthe junction of the first mesh layer 106 a and the semiconductor layer102, which in turn feeds back to the band bending and built in voltageV_(bi) at the junction. Because the electrolyte coupled to the gatevoltage V_(G) has direct access to the surfaces of the first mesh layer106 a and the semiconductor layer 102 due to the porosity of the firstmesh layer 106 a, the effect of the gate voltage V_(G) on the interfacedipole can be dramatic. At more positive gate voltage V_(G) biases, theinterface dipole is enhanced and contributes to the Schottky barrierheight manifesting itself as a reduced forward current in the firstquadrant (I). In the fourth quadrant (IV), the additional tunnelingbarrier due to this enhanced interface dipole increases recombinationlosses, manifesting itself as the reduced current “kink” 430. Furthercontributing to this barrier is an electric field induced across thedepletion layer due to the positive charges accumulated in the ionicliquid in direct contact with the Si surface, between nanotubes(generatng an electric field in a direction opposing the fieldassociated with the built-in potential). Switching to negative gatevoltage V_(G) biases reverses these trends.

As shown in FIGS. 4 and 5, the open circuit voltage V_(OC) saturates at0.55V with negative gate voltage V_(G). This saturation may be due to aregion of high surface state density, that once reached, preventsfurther change in the built-in potential V_(bi).

FIG. 6 is a perspective view of a nonlimiting embodiment of a solar cell100. The embodiment of a solar cell 100 illustrated in FIG. 6, denotedas solar cell 100 b, may be more useful for commercial applications thanthe embodiment illustrated in FIG. 1, denoted as solar cell 100 a.Nonetheless, the solar cell 100 b in FIG. 6 is similar to the solar cell100 a as will be discussed below, and the various materials discussedabove with respect to the solar cell 100 a may also be included in solarcell 100 b.

The solar cell 100 b includes a semiconductor layer 102 forming aSchottky junction 200 with an electrically conducting, opticallytransparent, porous mesh layer 106 a including nanotubes. Thesemiconductor layer 102 has a back contact layer 101, which becomes thenegative terminal of the solar cell 100 b. At the face of the solar cell100 b to be exposed to solar radiation, a first metallization layer 104a (patterned in the form of metal finger electrodes, also denoted hereinas finger electrodes 104 a) couples electrically to the nanotubes of thefirst mesh layer 106 a. The spacing between the finger electrodes 104 ais determined by the trade-off between the deleterious amount of lightthat additional such finger electrodes 104 a block from thesemiconductor surface, and the increase in the cell series resistancethat result as fewer such finger electrodes 104 a are used.

The finger electrodes 104 a are in turn coupled by thicker, but morewidely spaced, perpendicular metal bus bars 130 that couple all thefinger electrodes 104 a electrically. The spacing of the bus bars 130 isagain determined by the trade-off between the deleterious amount oflight that more such bus bars 130 would block from the surface of thesemiconductor layer 102 and the increase in the solar cell 100 b seriesresistance that results as fewer such bus bars 130 are used. Any one ormore of the metal bus bars 130 becomes the positive terminal of thesolar cell 100 b. Rather than overlie the first mesh layer 106 a themetal finger electrodes 104 a and the metal bus bars 130 may underliethe first mesh layer 106 a. The metal finger electrodes 104 a and busbars 130 may stick better to the surface of the semiconductor layer 102(or to a thin oxide layer that may underlie the metal finger electrodes104 a and bus bars 130 only) than they do to the nanotubes improving theintegrity of the solar cell 100 b. In that case the first mesh layer 106a, due to the flexibility of the nanotubes, will conform down to thesurface of the semiconductor layer 102 around the finger electrodes 104a still forming the nanotube-semiconductor junction. In either case(nanotubes under or over the finger electrodes 104 a and bus bars 130)the porous first mesh layer 106 a is saturated to the surface of thesemiconductor layer 102 with an optically transparent, ionicallyconducting material of the ionic conductor layer 108.

Proximate to the semiconductor layer 102, but not occupying any of itssurface “real estate,” is a can 134 within which lies a high surfacearea electrode 136 that is also saturated with the ionically conductingmaterial of the ionic conductor layer 108. The high surface areaelectrode 136 can be any number of materials to include activatedcarbon, or a pseudocapacitive electrode of the type used insupercapacitors such as, for example, manganese oxide or vanadium oxicor mixtures of these and/or other such materials possessing a high halfcell capacitance.

The ionic conductor layer 108 that saturates the first mesh layer 106 ais ionically coupled to the ionically conducting material in the can 134that saturates the high surface area electrode 136 via an ionicallyconducting bridge 132. A power supply 120 provides the small gatevoltage of about 0.75 V between the metal bus bar 130 and the highsurface area electrode 136 in a direction (the high surface areaelectrode made negative) that enhances the power conversion efficiencyof the solar cell 100 b. The power supply 120 that provides the gatevoltage can itself be a small solar cell.

In view of at least the foregoing discussion, the present applicationdescribes various embodiments of a solar cell 100 having a built-inpotential V_(bi) that can be adjusted by electronic gating (i.e.,changing a gate voltage V_(G)). Additionally, the electronic gating canmodify the interface dipole at the interface between the first meshlayer 106 a and the semiconductor layer 102 by partially emptying thesurface states and thereby increasing V_(bi), which enhances the solarcell 100 performance. Additionally, the ionic gating induces ions of theionic material to arrange themselves at the semiconductor surface. Forthe proper sign of the gating, these ions can contribute to the electricfield across the depletion layer in a direction that enhances the rateat which charge is swept out of the depletion layer thereby enhancingthe solar cell performance.

For semiconductors that possess an intrinsically high surface statedensity within the semiconductor bandgap, the increases in V_(bi) withgate voltage (of the appropriate sign) can saturate once the energy ofhigh surface state density is reached. Modification of the surface ofthe semiconductor layer 102 either before or after the contact with themesh layer 106 is made can modify the surface state density distributionto permit further increases in V_(bi) (with gate voltage of theappropriate sign) with concomitant enhancement of the solar cellperformance. Such modification can be induced mechanically, e.g., byabrasion, chemically or electrochemically by attachment of functionalgroups to the semiconductor surface, or by the deposition of speciesonto the surface by vapor or solution phase deposition. In the case ofvapor phase deposition, a plasma enhancement may be useful.

Referring next to FIG. 7A, shown is a perspective view of anothernonlimiting embodiment, among others, of a solar cell 100, denoted assolar cell 100 c. The solar cell 100 c includes a back contact layer 101and a semiconductor layer 102. The solar cell 100 c further includes aninsulating layer 103 on the semiconductor layer 102. A portion of theinsulating layer 103 is etched to expose the semiconductor layer 102,which is surrounded by a first metallization layer 104 a. Rather than afirst mesh layer 106 a as illustrated in FIG. 1, a grid layer 706 a isplaced on the etched portion such that the grid layer 706 a contacts thesemiconductor layer 102 and the first metallization layer 104 a. Thegrid layer 706 a is an electrically conducting grid that covers only afraction of the semiconductor layer 102 accessible through the etchedportion. For example, the grid layer 706 a may cover, about 50% of theexposed semiconductor layer 102, about 33% of the exposed semiconductorlayer 102, or about 25% of the exposed semiconductor layer 102. As canbe understood, other fractional coverage may be utilized. While theembodiments of FIG. 7A illustrates a rectangular grid layer 706 a, othergeometric grids may be utilized as can be understood.

The grid layer 706 a (and the mesh layer 106 b) ncludes one or more ofthe following materials: single wall carbon nanotubes, double wallcarbon nanotubes, multi-wall carbon nanotubes, graphene, semiconductingnanowires, metallic nanowires, a metallic grid or a semiconducting grid.Regardless of the material, the grid layer 706 a, which is in contactwith the semiconductor layer 102, may be sufficiently transparent to thesolar radiation that the major fraction of the radiation passing throughthe grid layer 706 a reaches the surface of the semiconductor layer 102.The grid layer 706 a is furthermore electrically conducting.Additionally, the grid layer 706 a may be sufficiently porous to permitaccess of the ionic conductor layer 108 to both the surface of thesemiconductor layer 102 and the portions of the grid layer 706 a thatcontact the surface of the semiconductor layer 102.

The solar cell 100 a further includes a second metallization layer 104 bon the insulating layer 103. A mesh layer 106 b contacts the secondmetallization layer 104 b. The grid layer 706 a and mesh layer 106 b areelectrically isolated from each other. The first metallization layer 104a forms a contact for the grid layer 706 a, and the second metallizationlayer 104 b forms a contact for the mesh layer 106 b. Also, the gridlayer 706 a contacts the semiconductor layer 102 whereas the mesh layer106 b is isolated from the semiconductor layer 102 by the insulatinglayer 103. The grid layer 706 a and first metallization layer 104 acollectively form a junction electrode. Similarly, the mesh layer 106 band the second metallization layer 104 b collectively form a gateelectrode.

FIG. 7B is a picture 700 of an example of a grid layer 706 a placed onan etched portion of the insulating layer 103 such that the grid layer706 a contacts the semiconductor layer 102 and the first metallizationlayer 104 a. For example, a gold contact layer 104 a with a 2×4 mm²rectangular window was evaporated onto a 1 μm thick oxide layer 103 onan n-Si wafer 102. The gold metallization layer 104 a was used as anetch mask to etch the oxide insulating layer 103 within the window downto the bare Si layer 102 surface. A 45 nm thick, 6×8 mm² rectangulararea thin, porous single wall carbon nanotube (SWNT) film wastransferred across the window contacting the gold and forming thejunction with the exposed n-Si semiconductor layer 102. This SWNT filmwas defined by standard photolithography and etching in oxygen plasma tocreate the grid pattern shown in FIG. 7B. The lines of the grid layer706 a are about 100 μm wide with about 300 μm spacing between adjacentlines. A second gold metallization layer 104 b and a SWNT mesh layer 106b were deposited on the oxide insulating layer 103 near the junction.This mesh layer 106 b and second metallization layer 104 b collectivelyform the gate electrode once the electrolyte was added.

Referring to FIG. 8, shown is a plot 800 illustrating cell currentdensity versus voltage (J-V) characteristic in the dark and underillumination (AM1.5G, 100 mW/cm²) for two distinct devices in theabsence of electrolyte (i.e., without ionic conductor layer 108). Afirst solar cell device utilizes a first SWNT mesh layer 106 a (FIG. 1)across the entire window (see curves 803) and the second solar celldevice includes a SWNT grid layer 706 a (FIG. 7A) limiting the Sicoverage with the nanotube lines to about 27% of the window area (seecurves 806). The nanotube film/n-Si contact forms a “conventional”metal-semiconductor Schottky junction solar cell. Fermi levelequilibration of the n-Si with the nanotubes, transfers electrons fromthe n-Si to the SWNTs generating a depletion layer and band bending inthe Si semiconductor layer 102, in the vicinity of the nanotubes in thegrid layer 706 a. Photons absorbed in the Si generate electron-holepairs that are separated by the built-in potential V_(bi), enablingpower generation from the solar cell device. For our doping density ofabout 10¹⁵ donors/cm³, this depletion layer extends ≦1 μm into the Sisemiconductor layer 102 from the contact with the nanotubes. Given therelatively small extent of this depletion layer, the reduced junctionarea of the grid layer 706 a yields a reduced short circuitphotocurrent. This reduction in the photocurrent does not scale indirect proportion to the reduced junction area because high qualitysingle crystal silicon has long diffusion lengths, allowingphotocarriers generated far from the junction to diffuse there andcontribute to the photocurrent. Nevertheless, the photocurrent in thedevice including the grid layer 706 a is reduced by more than a factorof two over that of the device including the first mesh layer 106 a,yielding a corresponding decrease in the full window-area-normalizedpower conversion efficiency.

With the addition of the 1-Ethyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide (EMI-BTI) ionic liquid (IL)electrolyte as the ionic conductor layer 108, the solar cell behaviorchanges significantly. FIG. 9 is a graph 900 that compares theilluminated J-V curves of a solar cell device utilizing a grid SWNTlayer 706 a (FIG. 7A) before and after addition of the IL electrolyte asthe ionic conductor layer 108, with the gate electrode electricallyfloating. Also shown in FIG. 9 is the case with −0.75 V applied to thegate electrode relative to the SWNT-grid junction electrode. The simpleaddition of the IL electrolyte (gate floating or not) more than recoversthe short circuit photocurrent lost to the reduced area coverage of then-Si by the nanotubes in the grid layer 706 a. The electrolyte inducesits own depletion (or inversion) layer in the Si semiconductor layer 102across the large gaps between the nanotube lines of the grid layer 706a.

The nanotube-electrolyte/n-Si solar cell 100 c is distinguished fromphotoelectrochemical cells in that there is no redox couple and theEMI-BTI electrolyte was selected for the ionic conductor layer 108because of its very broad electrochemical window ranging from −2.6V to+2.0 V (vs. Fc/Fc+i.e. centered at −5.1 V relative to the vacuum level).Thus, the EMI-BTI electrolyte does not participate in the chargetransport. Instead, photogenerated holes that make it to the electrolyteinduced inversion layer in the Si semiconductor layer 102 are trapped bythe electric field within the inversion layer and diffuse along it untilthey encounter a nanotube grid line where they are collected. Becausethe electric field, which accumulates holes at the surface, also repelselectrons, deleterious surface recombination is largely avoided.

The situation is reminiscent of so called “grating”metal-insulator-semiconductor (MIS) cells, where narrow metal lines (Alor Mg) on the front surface of p-type silicon collected electronstrapped by an inversion layer formed at the p-Si surface in the regionsbetween the widely spaced metal lines. The inversion layer in thosedevices was induced by positive charge trapped in a SiO layer grown onthe Si layer. In the present case, the gate voltage is certainly capableof inducing charge (of either sign depending on the polarity of thegate) adjacent to the surface of the n-Si but interestingly the highshort circuit current (V_(Bias)=0 V) seen in the grid layer of FIG. 9occurs immediately on introduction of the ionic liquid (IL) electrolyteas the ionic conductor layer 108. This implies that negative ionsaccumulate at the n-Si surface upon simple introduction of the ILelectrolyte.

To determine if such charge separation can be explained by the nativeelectrostatics, the system was modeled using the a device simulationpackage: Synopsys® TCAD Sentaurus. To simulate the effect of theelectrolyte, the program's ability to simulate dielectric coatings wasusing as proxy for the electrolyte. A dielectric coating was used withthe very large dielectric constant (∈=5000), i.e. the mobile free ionsof the electrolyte are replaced by a “dielectric” layer having a boundcharge possessing an extreme polarizablity (the free charge ofelectrolytes precludes definition of a real DC dielectric constant forthem so that the AC dielectric constants available in the literature arenot relevant). The value of ∈=5000 comes from the ratio of acharacteristic dielectric layer length (about 100 μm) relative to thatof the characteristic Debye layer dimension in the electrolyte (<20 nm).The simulation confirmed the formation of an inversion layer generatingan electric field upon addition of the electrolyte. FIG. 10A shows agraph 1000 of the experimental J-V curves for the grid layer solar celldevice 100 c (FIG. 7A) where the current density remains normalized tothe full window area.

FIG. 10B shows the simulation parameters and geometry (not to scale) fora cross-sectional slice through a SWNT grid line of 100 μm width, havingits long axis perpendicular to the page. The SWNT line is treated as asimple metal of constant work function φ_(CNT)=−4.9 eV (consistent withthat of nitric acid purified SWNTs). The gate electrode is a gold line(φ_(Au)=−5.1 eV) situated on a 1 μm thick SiO₂ dielectric (∈=3.9). Belowthe SWNT grid line lies the junction with the n-Si (φ_(Si)=−4.3 eV forthe 1×10¹⁵ cm⁻³ phosphorous doping density), which adjacent to the SWNTline is in direct contact with the “electrolyte” (dielectric with∈=5000) that coats the entire structure. The Neumann boundary conditionsused place mirror planes at the left and right sides of the figuremaking the gold gate electrode line, including its reflection on theleft side, 100 μm wide (equal area to the SWNT grid line) and thespacing to the next SWNT grid line, including the reflection on theright side, 300 μm.

FIGS. 10C, 10E, and 10G are graphical representations illustrating theelectric field developed in the depletion layer below the SWNT/n-Sijunction and in the adjacent n-Si at a bias voltage V_(Bias)=0 V forgate voltages: V_(Gate)=−0.75, 0, +0.75 V (1003 c, 1003 e, and 1003 g ofFIGS. 10C, 10E, and 10G, respectively). FIGS. 10D, 10F, and 10H aregraphical representations illustrating this at a forward bias voltageV_(Bias)=0.3V for the same gate voltages: (1003 d, 1003 f, and 1003 h ofFIGS. 10D, 10F, and 10H, respectively). These results clearly show anelectric field 1003 at the silicon surface that extends over the long300 μm spacing between the neighboring SWNT grid lines. The simulationmoreover shows that this field 1003 decreases as V_(Gate) progressivelyincreases from −0.75, to +0.75 V (FIGS. 10C to 10H), consistent with thesmaller measured currents at V_(Gate)=+0.75V and V_(Bias)=0V as shown inFIG. 10A.

The simulation is focused on the inversion layer generated by theelectrolyte and no attempt was made to model the other gate fielddependent features of the J-V curves. Thus, the model does not forexample include the gate induced shift in the SWNT Fermi-level. Nor doesit incude the resistivity changes in the SWNT film, irrelevant to theelectrostatics. Nevertheless, the model captures the existence of aninversion layer extending well beyond the direct depletion layer in thevicinity of the SWNT/n-Si contact and thus explains the increasedsaturation currents upon addition of the IL electrolyte.

This behavior can be qualitatively understood as follows. When thenanotube grid layer 706 a and the n-Si semiconductor layer 102 are firstplaced in intimate contact the free energy of electrons in the n-Si(work function: φ_(Si)=−4.3 eV) is reduced by their transfer to thecarbon nanotubes (work function: φ_(CNT)=−4.9 eV). Such transfer stopswhen Coulombic restoring forces due to the charge imbalance raise thelocal potential (the built in potential V_(bi)) to prevent furthercharge exchange, establishing equilibrium. In the presence ofelectrolyte ions, the ions are free to migrate to compensate thetransferred charge and thus permit the exchange of substantially morecharge before the equilibrium is reached. Additional electrons aretransferred to the nanotubes from the Si regions between the nanotubegrid lines of the grid layer 706 a compensated by positive electrolyteions surrounding the nanotubes, while the positive charge left behind inthe n-Si inversion layer is compensated by negative electrolyte ionsaccumulated at the Si layer 102 surface. The electrolyte here servesmuch as it does in an electrolytic capacitor to raise the capacitance ofthe system with a self potential provided internally by the originalFermi level offset between the nanotubes and the n-Si, or externally bythe gate field. These results also prompt further consideration of thecause(s) of the increasing energy gap-like feature (observed near theV_(OC)) with increasing gate voltage.

Once charge reorganization in the electrolyte of the ionic conductorlayer 108 due to an applied gate field is complete, the gate circuitdraws negligble current and so consumes negligible power. Hence the gatefield enhancement of this inversion layer between the nanotube gridlines of the grid layer 706 a has little energetic penaty. Indeed,compared to the first mesh layer 106 a (FIG. 1) case, it has benefit.Photons in transit to the Si semiconductor layer 102 surface that areabsorbed in the SWNT mesh layer 106 a do not contribute to the powergeneration. This has been confirmed by placing a filter in the lightpath that only transmits light energies belowthe silicon bandgap. If thenanotubes participated in the photogeneratbn, the semiconductingnanotubeswith bandgaps of about 0.6 eV should have yielded somephotocurrent, however none was observed. Since they don't contribute tothe power generated, thinner nanotube films would absorb less light,allowing transmission of more power to the silicon, thereby enhancingthe PCE. However, thinning the nanotube films increases their resistancein a non-linear fashion, introducing detrimental series resistance. Theability to use a liquid junction and reduce the area of the Sisemiconductor layer 102 that must be covered by the nanotube mesh layer106 a suggested that a grid pattern of optimized spacing could minimizethe overall absorptive losses while minimally increasing the seriesresistance, yielding a net gain in the PCE. This turns out to be thecase. The SWNT mesh layer 106 a achieved a best PCE of 10.9% at aV_(Gate)=−0.75 V. At this same gate voltage, the grid layer 706 a havingthe geometry shown in FIGS. 7A-7B attains a PCE of 12%, an improvementof 10% over the mesh layer 106 a configuration of FIG. 1.

Compared to the case of a continuous film, the increased solar fluxarriving at the electrolyte/n-Si junction of the grid cell shouldmanifest itself as a larger short circuit current density. The shortcircuit current density for the mesh layer 106 a was J_(scm)=25.0mA/cm², while that for the grid layer 706 a was a larger J_(SCG)=29.8mA/cm², as expected. Furthermore, if the only difference in the shortcircuit current of a device with a grid layer 706 a and a device with amesh layer 106 a (both coated with IL electrolyte) lies in theabsorptive losses due to the relative areas covered by the nanotubes, itshould be possible to semi-empirically calculate the ratio of thecurrent densities obtained in the two cases from considerations of onlythe relative solar power absorbed within the silicon in the two cases.The latter can be determined from measurements of the reflectance ofsilicon, the reflectance of a nanotube film on silicon and thetransmittance of a nanotube film, on glass (ionic liquid coated in allcases) recorded over the relevant range of the solar spectrum (300nm-1107 nm). The ratio of the measured current densities isJ_(SCG)/J_(scM)=29.8 mA/25 mA=1.19, while the calculation yields 1.17which is excellent agreement given no adjustable parameters and errorsto be expected from neglect of the second transit through the nanotubesthat appears in the reflectance measurement, and neglect of thereflection from the front surface of the nanotube film/ionic liquidoccurring in the transmittance measurement. Reassuringly, correction forthese effects would raise the calculated value, further improving theagreement.

Although the PCE of grating MIS cells could exceed 17%, it was foundthat they degraded rather drastically with time. The degradation wastraced to electrons that accumulated from the environment at the SiOlayer surface. Because this charge tended to neutralize the trappedpositive charge in the SiO layer (responsible for generating theinversion layer that permitted the wide electrode spacing) the magnitudeof the inversion layer decreased, degrading the solar cell performance.The use of electrolytes, which are intrinsically neutral and induce theinversion layer via a spontaneous charge separation, may eliminate thisproblem. As is seen in the data of FIG. 9, simple addition of theelectrolyte as the ionic conductor layer 108, even with the gateelectrically floating, yields a short circuit photocurrent equal to thesaturation photocurrent implying existence of the inversion layer evenbefore any gate field is applied. In the case of SWNTs, the appearanceof the gap like feature on electrolyte addition reduces the fill factorso that the gate field is necessary to achieve the maximum powerconversion efficiency. Such gap like feature is not anticipated in thecase of conventional metal electrodes. In that case, a grating MISSchottky junction solar cell with the SiO layer replaced by the ionicliquid electrolyte may provide the solution to the previous degradationproblem even without the need for active gating (while gating incurslittle energetic penalty, it does add to the device complexity).Finally, such solar cells may also benefit from a texturing of the Sisemiconductor layer 102 to trap more of the light that would otherwisebe reflected from its surface. Recently arrays of nanoholes in a p-njunction Si solar cell have been demonstrated to enhance the deviceperformance. The large spacing permitted between the grid lines of thegrid layer 706 a in the electrolyte coated solar cell 100 c indicatesthat substantial benefits of both the inversion layer and the additionallight trapping may be achieved by filling the nanoholes withelectrolyte.

It should be emphasized that the above-described embodiments are merelypossible examples of implementations set forth for a clear understandingof the principles of the disclosure. Many variations and modificationsmay be made to the above-described embodiments without departingsubstantially from the spirit and principles of the disclosure. All suchmodifications and variations are intended to be included herein withinthe scope of this disclosure and the present application.

It should be noted that ratios, concentrations, amounts, and othernumerical data may be expressed herein in a range format. It is to beunderstood that such a range format is used for convenience and brevity,and thus, should be interpreted in a flexible manner to include not onlythe numerical values explicitly recited as the limits of the range, butalso to include all the individual numerical values or sub-rangesencompassed within that range as if each numerical value and sub-rangeis explicitly recited. To illustrate, a concentration range of “about0.1% to about 5%” should be interpreted to include not only theexplicitly recited concentration of about 0.1 wt % to about 5 wt %, butalso include individual no/concentrations (e.g., 1%, 2%, 3%, and 4%) andthe sub-ranges (e.g., 0.5%, 1.1%, 2.2%, 3.3%, and 4.4%) within theindicated range. The term “about” can include traditional roundingaccording to significant figures of numerical values. In addition, thephrase “about ‘x’ to ‘y’” includes “about ‘x’ to about ‘y’”.

The invention claimed is:
 1. A solar cell comprising: a semiconductorlayer having a first side and a second side; a first mesh layer formedon the first side of the semiconductor layer, wherein the first meshlayer includes a porous mesh of nanotubes; a first metallization layerin direct contact with at least a portion of the first mesh layer; anelectrode coupled to the first metallization layer by a gate voltagesource; a second metallization layer disposed on the second side of thesemiconductor layer; and an ionic layer, wherein the electrode isphysically separated from the first metallization layer and the secondmetallization layer, such that the electrode is in electricalcommunication with the first mesh layer via the ionic layer.
 2. Thesolar cell of claim 1, wherein a built-in potential of a junction of thefirst mesh layer and the semiconductor layer is responsive to a voltageapplied by the gate voltage source.
 3. The solar cell of claim 1,wherein an interface dipole a junction of the first mesh layer and thesemiconductor layer is responsive to a voltage applied by the gatevoltage source.
 4. The solar cell of claim 1, wherein the gate voltagesource includes a plurality of solar cells.
 5. The solar cell of claim1, wherein the first metallization layer forms a first electrode and theelectrode forms a second electrode isolated from the semiconductorlayer.
 6. The solar cell of claim 1, wherein the first metallizationlayer is insulated from the semiconductor layer by an insulating layer.7. The solar cell of claim 1, wherein the first metallization layerdirectly contacts the semiconductor layer.
 8. The solar cell of claim 1,wherein the first mesh layer includes a graphene layer.
 9. The solarcell of claim 1, wherein the first mesh layer includes semiconductingnanowires.
 10. The solar cell of claim 1, wherein the first mesh layerincludes metallic nanowires.
 11. The solar cell of claim 1, wherein aseries resistance of the solar cell is responsive to a voltage appliedby the gate voltage source.
 12. The solar cell of claim 1, wherein thefirst mesh layer includes a metallic grid.
 13. The solar cell of claim1, wherein the first mesh layer includes a semiconductor grid.
 14. Asolar cell comprising: an article, comprising: a first mesh layercomprising a porous mesh of nanotubes; a semiconductor layer, wherein atleast a portion of the semiconductor layer is positioned to form aSchottky junction with at least a portion of the first mesh layer; afirst metallization layer electrically coupled to the first mesh layer;a second metallization layer electrically coupled to the semiconductorlayer; and an ionic conductor layer; and an electrode physicallyseparated from the article, wherein the electrode is electricallycoupled to the first metallization layer or the second metallizationlayer by a voltage source, and wherein the electrode is coupled to thefirst mesh layer and/or the semiconductor layer by the ionic conductorlayer.
 15. The solar cell of claim 14, wherein the ionic conductor layerdoes not participate in reduction-oxidation reactions during operationof the solar cell.
 16. The solar cell of claim 14, wherein a surface ofthe semiconductor layer has been modified to change a surface statedensity, thereby increasing a range of a built-in potential responsiveto a voltage applied by the voltage source.
 17. The solar cell of claim14, wherein the solar cell generates power when a surface of thesemiconductor layer is illuminated by solar radiation.
 18. The solarcell of claim 14, wherein the first mesh layer is transparent.
 19. Thesolar cell of claim 14, wherein the first mesh layer is sufficientlyporous to permit access of the ionic conductor layer to both a surfaceof the semiconductor layer and portions of the first mesh layer thatcontact the surface of the semiconductor layer.
 20. The solar cell ofclaim 16, wherein the semiconductor surface has been modified after thefirst mesh layer is formed on the semiconductor layer.